The present invention relates to a process for the preparation of polyarylene sulfide.
Polyarylene sulfide (hereinafter referred to as PAS), typically polyphenylene sulfide (hereinafter referred to as PPS), has been widely used as a material for electric and electronic parts, automobile parts and chemical apparatus parts because it is excellent in heat resistance, processability, chemical resistance, flame resistance and dimensional stability. Particularly, so-called linear PAS, which is made to have high molecular weight by polymerization has such characteristics as less impurity content, good hue and excellent heat stability. It has, therefore, high value for use.
Some processes for the preparation of the aforesaid linear PAS are known, such as a process in which an alkali metal carboxylate is used as a polymerization aid in a polymerization reaction (Japanese Patent Publication No. Sho-52-12240/1977), a process in which the polymerization reaction is carried out in two steps and a large amount of water is positively added in the second step reaction (Japanese Patent Application Laid-Open No. Sho-61-7332/1986), or a process in which a part of a gaseous phase in a reactor is condensed by cooling a gaseous phase part of the reactor to return the condensed liquid to a liquid phase during the polymerization reaction (Japanese Patent Application Laid-Open No. Hei-5-222196/1993).
However, when the PAS obtained by these processes is used in injection molding, crystallization is sometimes insufficient depending on molding conditions. In order to shorten a molding cycle, it is needed that a crystallization speed of PAS is high, that is, a crystallization temperature, Tc, is high. However, the PAS obtained by the aforesaid processes has a low Tc and, therefore, its molding cycle cannot be shorten.
As a method for raising the crystallization temperature, Tc, Japanese Patent Application Laid-Open No. Sho-62-48728/1987, for instance, discloses a process where a formed PAS polymer is separated from a polymerization reaction mixture after the polymerization and treated in a strong acid solution of a pH less than 2. In Japanese Patent Application Laid-Open No. Hei-7-118389/1995, there is disclosed a process for the acid treatment of PAS in an organic solvent/water mixture containing an organic acid in a concentration of 0.1-5.0 % by weight with a weight ratio of the organic solvent : water being in a range of 4:1 to 1:10. However, in both of these methods, PAS is treated in an acid solution of a low pH and, therefore, these methods have a disadvantage that the filterability (filtration rate) of the reaction slurry is poor and, accordingly, the productivity is low. Moreover, it has been found that in the process of Sho-62-48728/1987, a melt viscosity of the PAS obtained is lowered remarkably by the acid treatment. In the process of Hei-7-118389/1995, a cost for recovery of the solvent is high because the filtrate after the water washing contains water and by-product salts as well as the solvent.
In Japanese Patent Publication No. Hei-6-68025/1994, there is disclosed a process in which an inorganic or organic acid is added to a PAS slurry after the completion of a polymerization and the slurry is stirred at or below pH 6, washed, filtered, washed with water and dried. The aforesaid process relates to a purification process to remove impurities such as alkali metal ions in PAS. In the PAS obtained by this process, the impurity content is not sufficiently low and a crystallization temperature, Tc, is low.
In all of the acid treatment processes mentioned above, PAS is separated by filtering the PAS slurry obtained and then repeatedly washed with water and filtered. Accordingly, almost all of the solvent used in the polymerization is removed from PAS by the washing with water and filtration and thus contained in the filtrate. It is, therefore, necessary to recover the solvent. Because the filtrate is composed of water, the polymerization solvent and by-product salts, the recovery of the solvent is complicated and costly. Meanwhile, if the solvent is not recovered, the cost increases inevitably. This is not economical, either. When PAS is fine powder, filterability is low in the washing with water and filtration and, therefore, it takes long time for filtration. This leads to low productivity.
Meanwhile, PAS is of pale yellow in most cases. Even when PAS of high whiteness is obtained, it is easily colored in melt molding. Accordingly, in order to obtain PAS articles of high whiteness, special means are needed. Various means for decreasing discoloration have been used.
In Japanese Patent Application Laid-Open No. Sho-60-8359/1985, it is proposed that white pigment is mixed to a PPS resin to whiten it while a decrease in mechanical strength due to the addition of the white pigment is compensated by adding an epoxy resin. However, this leads to higher costs. Japanese Patent Application Laid-Open Nos. Hei-3-28267/1991 and Hei-3-28268/1991 discloses a composition which is obtained by adding an organic phosphorus compound to PAS. However, there are disadvantages that a melt viscosity increases and, in addition, gas generates in melt molding.
In Japanese Patent Application Laid-Open No. Hei-7-70320/1995, the present inventors discloses a process for the preparation of PAS by reacting an alkali metal sulfide with a dihaloaromatic compound in a polar aprotic solvent, wherein 0.001-0.1 mole of a zinc compound per mole of the alkali metal sulfide is added to a polymerization system or an aftertreatment system at any point of time from a time when a conversion of the dihaloaromatic compound is 30 % to a step of aftertreatment of a polymerization slurry. This process provides PAS with high whiteness and high melt viscosity.